High voltage divider unit

ABSTRACT

A resistive voltage divider instrument is disclosed which permits very accurate measurements to be taken of the potential of high voltage DC sources, with low DC current drain, by means of a sensitive low voltage differential voltmeter. This voltage divider embodies an instrument packaging concept that utilizes a special insulation system to enclose resistor strings formed of precision wire wound high voltage resistors. The resulting insulated resistor modules are interconnected and positioned within a molded plastic case in such a manner as to minimize all voltage gradients between resistor modules and the ground plane. Special provisions are made for preventing the formation of corona and for minimizing the effects of leakage current errors. Each resistor module contains a series connected string of accurately matched wire wound precision resistors and is filled with an insulating material that is highly resistant to the passage of leakage currents through its volume or across its surface. All interconnections between modules and the connections to the input voltage terminals located at the back of the instrument case are made with special connector and lead assemblies that are resistant to high voltage corona and that provide a quick disconnect feature.

United States Patet n 1 Till [54] HIGH VOLTAGE DIVIDER UNIT James PeterTill, Camp Hill, Pa. [73] Assignee: AMP Incorporated, Harrisburg, Pa.221 Filed: Dec. 17', 1969 21 App1.No.: 885,788

[75] Inventor:

FOREIGN PATENTS OR APPLICATIONS 1,123,566 9/1956 France ..324/l22 OTHERPUBLICATIONS Hague, BL; Instrument Transformers; book pub. by Pitman.&Sons, London; 1936; pg. 366-369.

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-Ernest F. KarlsenAtt0rneyCurtis, Morris and Safford, William J. Keat- COM 5 KV Jan. 9,1973 ing, RonaldrD. Grefe, William Hintze, Adrian J. La Rue, FrederickW. Rating, Jay L. Seitchik and John P. Vandenburg [57] ABSTRACT Aresistive voltage divider instrument is disclosed which permits veryaccurate measurements to be taken of the potential of high voltage DCsources, with low DC current drain, by means of a sensitive low voltagedifferential voltmeter. This voltage divider embodies an instrumentpackaging concept that utilizes a special insulation system to encloseresistor strings formed of precision wire wound high voltage resistors.The resulting insulated resistor modules are intercon-' nected andpositioned within a molded plastic case in such a manner as to minimizeall voltage gradients between resistor modules and the ground plane.Special provisions are made for preventing the formation of corona andfor minimizing the effects of leakage current errors. Each resistormodule contains a series connected string of accurately matched wirewound precision resistors and is filled with an insulating material thatis highly resistant to the passage of leakage currents through itsvolume or across its surface. All interconnections between modules andthe connections to the input voltage terminals located at the back ofthe instrument case are made with special connector and lead assembliesthat are resistant to high voltage corona and that provide a quickdisconnect feature.

4 Claims, 11 Drawing Figures PAIENTEDJAu slsza SHEET 1 0F 6 INVENTOR.JAMES PETER TILL BY I ATENTEDJAI 9 I975 3,710 52 1. RI? l I RI5 o VOLT RI COM I i0) RM 4 85 86] [9O Rn 62 Q R|2 I 40? 8 I 42 66 I 0 v \1 82METER PATENTED JAN- 9 I973 SHEEI 5 [IF 6 HIGH VOLTAGE DIVIDER UNITBACKGROUND OF THE INVENTION Accurate measurements of DC voltages abovevolts are usually made with the aid of a resistive voltage dividercircuit which serves to reduce the magnitude of the voltage applied tothe measuring device. The voltage divider in question comprises a highresistance R in series with a low resistance R The voltage under test isapplied across the series combination with R being connected at thegrounded end. The divider ratio, which may be expressed as R R /R ischosen to produce a specific voltage output across R,, which can then bedetermined accurately with a null type differential voltmeter or apotentiometric device. This technique is the well-known volt-box methodwhich is useful for measuring source voltages up to no more than 1,500volts. Attempts to extend the volt-box method to still higher voltageshave failed due to the difficulties encountered in constructing a highvoltage divider which would have a constant effective resistance value;i.e. a resistance value that does not change as the applied voltage isvaried.

Variation of the effective resistance value with applied voltage may becaused by any one or a combination of at least three basic factors. Thefirst factor is the heating of the resistor wire due to PR losses. Theamount of the resulting change in resistance will be dependent upon theresultant temperature coefficient of the entire resistor moduleassembly. The second is the leakage current through the volume and/oracross the surface of the insulation or insulations used to support andprotect the individual resistors. This type of leakage increases withincreasing applied voltages and effectively shunts and decreases totalresistance. Finally, there are corona discharges which tend to appear atany location along the resistor module assembly having a high gradient;such corona discharges effectively leak a part of the resistor currentto ground.

The factor of resistor wire heating was overcome first by selectingbasic resistors which have a low temperature coefficient, and then bymatching them within individual resistor modules so that half havepositive temperature coefficients and the other half have negativecoefficients. This arrangement will reduce the overall temperaturecoefficient to a negligible minimum.

The other two factors involving leakage current and corona dischargecannot be reduced to an acceptably low magnitude in any suitable manner.In fact it is difficult to even determine or measure their magnitude.One solution known to the prior art and widely followed was to provide avery large number of individually shielded precision resistors, forexample 100 or 200 one-megohm units. These resistors were connected inseries and arranged in a vertical helix array between ground and a highvoltage electrode structure. See Special Shielded Resistor forHigh-Voltage D-C Measurements" by J.H. Parks, dated Sept. 26, 1961, andpublished in the Journal of Research of the National Bureau ofStandards-C. Engineering and Instrumentation, Vol. 66C., No. l,January-March, 1962.

The use by Park of individual shields prevented the formation of coronaor leakage currents at the surface of any resistance element no matterhow high the potential of the shields above ground. The Park arrangementeffectively provided a uniform leakage current path around each resistorto ground; also the vertical helical configuration of the resistorstring with a large high voltage electrode at the top served to preventconcentration of electric field and corona potential at the high voltageend of the divider. Tests showed that corona and leakage errors for thisresistance divider were less than 10 parts per million at 50 kilovolts.

However, there are a number of serious drawbacks associated with thePark resistance divider. In the first place, the Park divider is of suchlarge physical size that it has proved to be too cumbersome or unwieldyto serve as a portable test instrument for field use. Also itsconstruction, requiring a large number of special milled metal parts andlarge numbers of individual resistors, is such as to result in aninstrument that is very expensive to build. Finally, no safety featureswere present to protect users of the divider from the dangers of highvoltage in conditions of use outside the laboratory environment.

SUMMARY OF THE INVENTION The present invention relates to a novel andimproved resistance divider instrument for use in measuring high DCvoltages.

It is an object of the invention to provide a high voltage dividerinstrument for measuring high voltage which is compact in size and lightin weight so as to form a truly portable instrument suited to field use.Another object is to provide a resistive voltage divider overcoming thedrawbacks of the prior art units specified above. A further object is toprovide a high voltage divider unit that greatly reduces the cost ofconstruction. Another object of the invention is to provide a voltagedivider arranged to accept a plurality of levels of high voltage inputvalues, and further arranged so that the user may select any one of aplurality of division ratios and thereby obtain a desired level ofoutput voltage. It is another object to provide a high voltage dividerthat is completely safe for the user, which does not have any exposedhigh voltage points, and which permits adjustment of the variouscontrols in safety even with the high voltage on.

The foregoing objects are attained by the invention through thesuppression of the effects of corona and leakage current by employinginsulating materials which exhibit very high surface and volumeresistivity. Relatively small numbers of precision wire wound highresistance resistors are packaged into modules of special constructionto form the high voltage input section of the divider. A plurality ofsuch resistor modules are geometrically positioned in a particularmanner from the high voltage input points to ground to form a dividerassembly which minimizes the adverse effects of high voltage gradientsbetween resistor modulesand from any given module to ground or to a highvoltage input. Tests made on the divider instrument of the presentinvention proved corona and leakage current error to be extremely lowand in the vicinity of only two parts per million at input voltages upto as high as 20 kilovolts. The present divider also features specialhigh voltage leak and connectors for safety purposes. The end resultachieved is a completely enclosed system with no exposed high voltagedanger points.

The foregoing and other objects, features and advantages of theinvention will be better understood from the following detaileddescription when considered together with the drawings and claims.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a perspective view of thehigh voltage di-- vider instrument of the invention in use inconjunction with a differential voltmeter;

FIG. 2a is a side view, partly in section, of the high voltage dividerunit;

FIG. 2b is a view, from the rear, of aportion of the unit showing theinput voltage terminals;

FIG. 3 is a schematic circuit diagram of the high voltage divider of thepresent invention;

FIG. 4 is an exploded perspective view depicting the construction of oneof the resistor modules employed in the high voltage divider;

FIG. 5 is a perspective view in section of the resistor module of FIG. 4when assembled;

FIG. 6 is an exploded perspective view, similar to FIG. 4, of anotherresistor module used in the high voltage divider;

FIG. 7 is a perspective view, comparable to FIG. 5, of the resistormodule of FIG. 6, as assembled;

FIG. 8 is a perspective view of a portion of the high voltage dividerunit, illustrating the manner in which the resistor modules of FIGS. 4-7are mounted thereon, together with the circuit connections required; and

FIGS. 9 and 10 are simplified circuit diagrams useful in explaining theoperation of the high voltage divider of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description,reference is made to specific materials, dimensions, circuit parameters,components, etc., which are being given only by way of example. The truescope of the invention is defined in 'the appended claims.

Referring now to FIG. 1, the high voltage divider unit of the inventionis indicated generally at 30, and is shown suitably connected to aconventional DC differential voltmeter 32, such, for example as a ModelNo. 891-A solid state DC Differential Voltmeter available from the JohnFluke Mfg. C0., P.O. Box 7428, Seattle, Wash. 98133. The combination ofthe precision voltage divider 30 and the sensitive low voltage voltmeter32 may be used to obtain an accurate measurement of the unknownpotential of a high voltage DC source, not shown. It is the purpose ofthe present high voltage divider 30 to allow measurement of highvoltages, up to or kilovolts or more, with accurate low voltagevoltmeters such as the Fluke meter 32, while imposing only minimumloading effects on the voltage source under test. The unit is aresistance divider constructed to obtain accurate voltage division ofsuch high voltages in either positive or negative polarity.

The voltage divider 30 receives the high voltage from the source to bemeasured via a cable 34 connected to the appropriate one of its pluralinput terminals (see FIG. 2b); voltage divider 30 in turn has its outputconnected to the precision voltmeter 32 by a lead 36, with the lead 38providing a ground connection. A ratio selector switch 40 controls therange or nominalmagnitude of the DC output voltage of divider 30, whilean auxiliary voltmeter 42 located within the divider 30 permits anapproximate initial determination of the magnitude of the appliedinputvoltage so that the correct input terminal (see FIG. 2b) may beemployed.

Referring to the schematic circuit diagram shown in FIG. 3, the presenthigh voltage divider unit comprises an input section, seen across thetop of the figure, and an output section which encompasses the restofthe figure. There are six separate resistor modules, identified as 1A,13, 2A, 28, 3A and 3B, which are connected together in series to make upthe input section of the instrument. The details of the construction ofthe resistor modules will be considered later. In the illustratedembodiment, the divider is arranged to accept input voltages up to andincluding 15 kilovolts. Three input terminals 44, 46, and 48 areprovided, with inputs up to 5 kilovolts being imposed on terminal 44.Inputs between 5 and 10 kilovolts will be applied to terminal 46 andthose between 10 and 15 kilovolts to terminal 48, the one which isconnected to one end of resistor module IA. Input terminal 46 and 44 areconnected, respectively, to the junction of modules 13 and 2A and thejunction of modules 2B and 3A as shown.

The output section consists of a series connected resistance stringincluding adjustablev potentiometers R- 19, R-l6, and R-l3 together withfixed resistors R-18, R-l5, and R-l2, relatively positioned andinterconnected in the manner shown in FIG. 3. A protective resistor R-20is connected in shunt across potentiometer R-19 to ensure circuitcontinuity in the event of a fault causing an open circuit in thepotentiometer. The other potentiometers R-16 and R-13 are provided withsimilar protective shunt resistors R-17 and R-l4. The said outputresistance string is connected from one end of resistor module 3B to acenter terminal 62 of the ratio selector switch 40. The latter switchmay comprise a rotary switch having two decks as shown, with two centerterminals 60 and 62, a pair of rotatable contact arms 64 and 66 gangedtogether, and corresponding sets of six selectable switch terminals71-76 and 81-86. The voltage divider instrument 30 has three separatelow voltage output taps 52, 54, and 56 in order to provide any one ofthree different levels or ranges of output voltage, depending on thesetting of switch 40. The tap 52 provides for an output voltage level of100 volts, while taps 54 and 56 serve as outputs for levels 'of 10 voltsand 1 volt, respectively. Output tap 52 is connected to the upper end ofpotentiometer R-l9, tap 54 is coupled to the junction of R-18 and R-l6,and the remaining tap 56 is connected to the junction of R-15 and R-l3.

The first or upper deck of ratio selector switch 40, see FIG. 3, isemployed in a manner to permit the user to selectively connect any oneof the taps 52, 54, 56 to the'final output terminal 80 of the dividerunit 30. Thus in the upper deck of rotary switch 40, the switchterminals 76 and are unused; the switch terminals 74, 73, and 72 areconnected, respectively, to taps 52, 54 and 56; and the switch terminal71 corresponds to the Meter position of switch 40, which is also unusedin the upper deck. The center terminal 60 of the said upper deck is, ofcourse, connected directly to the unit output terminal 80.

Referring now to the second or lower deck of switch 40, the centerterminal 62 thereof is directly connected to R42 at the lower end of theoutput resistance string, as has been stated. The switch terminal 86 isconnected to the common return conductor 90, as shown, in FIG. 3 and theswitch terminal 85 is unused. Switch terminals 84, 83 and 82 aredirectly connected together as shown, and a resistor R- is connectedbetween 84 and the common return at 90. The switch terminal 81 in thelower deck serves as the Meter position; it is connected to one side ofthe auxiliary voltmeter 42, the other side of the meter being connectedto the common conductor at 90. Finally, a meter protecting resistor R- 1l is connected from 62 to 86 and thus is permanently shunted across thevoltmeter 42.

In FIG. 9 there is shown the schematic circuit diagram for resistivevoltage dividers in general, and in FIG. 10 appears a simplified orequivalent circuit diagram of the present divider 30. Thus FIG. 10corresponds to FIG. 3 in that resistor R-6 is the equivalent resistancefor the combined resistances of modules 1A and 1B; R-5 and R-4 similarlyare equivalents for module pairs 2A-2B and 3A-3B, respectively; resistorR-3 is the equivalent resistance for the parallel-series combination ofR--R-l9-R-18; resistor R-2 represents elements R-l7-R-16-R-15; andresistor R-l represents R-l4-R-13-R-l2 together with the equivalent ofR-l 1-R10 which are connected in parallel at positions 82, 83, 84 ofswitch 40. Considering FIG. 9, the division ratio D of the voltagedivider is defined by:

Correspondingly, the multiplier M of the circuit is given by:

The multiplier M is the more convenient factor for use when makingactual measurements of unknown input voltages, since with the unknowninput E applied to one of the input terminals 44, 46 or 48, the user hasonly to read E from differential voltmeter 32 and then obtain E from therelation:

The same relationships apply to the present divider 30 as represented inFIG. 10, but now the values of R and R,, are determined by the selectionmade of a particular input terminal and a particular output terminal ina given case. This is best seen from considering specific examples: (1)if the input is to the 15 kilovolt terminal 48 and the output is takenfrom the 1 volt tap 56, then R R4 and R R-2 R-3 R4 R-S R-6; (2) if, onthe other hand, the input is applied to the 5 kilovolt tap 44 and theoutput is taken from the 100 volt tap 52, then R,, R-l R-2 R-3 and RR-4, in this case R- 5 and R-6 not being used.

The voltage divider 30 is arranged to provide the following multiplierfactors M, as appears in Table 1:

TABLE 1 Max. Input Voltage Multipliers M 15 KV 15,000 1500 150 10 KV10,000 1000 100 5 KV 5,000 500 50 Max. Output Voltage 1 volt 10 volts100 volts Therefore, the voltage divider 30 is constructed to providethe following set of resistance values, with respect to the equivalentcircuit of FIG. 10, which results in a divider having the multipliers Mof Table 1:

TABLE 2 ELEMENT RESISTANCE R-6 50 megohms R-S 50 megohms R-4 49 megohmsR-3 900,000 ohms R-2 90,000 ohms R-l 10,000 ohms Where the instrument 30is intended to be accurate to within i 0.01 percent, the actualresistance parameters required for the circuit, referring to FIG. 3, aregiven in the following table:

TABLE 3 ELEMENT RESISTANCE Modules 1A 18 49.990 megohm Modules 2A 2849.990 megohm Modules 3A 3B 49.000 megohm R-20 10,000 ohms R49 10,000ohms R-l8 897,500 ohms R-17 1,000 R 16 1,000 R-15 89,750 ohms R-l4 1,000ohms R-13 ohms R-12 8,965 ohms R-l 1 1,000 ohms R-10 100,000 ohms Theresistive input impedance of divider 30, input terminal to commonterminal, is as follows:

TABLE 4 INPUT TERMINAL INPUT IMPEDANCE l5 KV (48) megohm i 0.01% 10 KV(46) 100 megohm i 0.01% 5 KV (44) 50 megohm i 0.01%

In the operation of the high voltage divider instrument 30, the sourceof the voltage to be measured should first be de-energized, for safety.The cable 34 from the voltage source is to be initially connected to the15 KV input terminal 48. This presents a minimum loading effect on thesource and minimizes the chances of damaging the divider due toovervoltage. Next the ratio selector switch 40 is set at its Meterposition (see FIGS. 3 arms 64 and 66 set to switch terminals 71 and 81)and the voltage source is energized. This places the auxiliary voltmeter42 in series with the output resistance string and applied a reducedvoltage proportional to the input voltage to meter 42. The user will nowobtain an approximate initial measurement of the unknown input by thereading taken from meter 42, which will allow him to determine whatinput range or input terminal to use for the final accurate reading tobe obtained with differential voltmeter 32. The auxiliary meter 42 alsoprovides an indication of the polarity of the input voltage.

The voltage source should be de-energized again, and cable 34 connectedto the appropriate input terminal 44, 46 or 48. Of course, for inputvoltages in the range of 0-5 KVDC, terminal 44 is to be used; for inputsfrom 5 KVDC to 10 KVDC, terminal 46 is used; and terminal 48 is used forinputs in the range of 10 KVDC I5 KVDC. Next, the desired output voltagerange is chosen, and ratio selector switch 40 is moved to thecorresponding position. For instance, if it is desired to read theoutput voltage on a 0-100 volt range on meter 32, then switch 40 is setin the position with arms 64 and 66 engaging switch terminal 74 and 84.The unknown input source is again energized, a voltage reading is takenfrom meter 32, and the meter reading is converted to the true valueusing. the appropriate multiplier M in accordance with Table 1. Thecontents of Table 1 appear on the front of the divider instrument itselfas indicated in FIG. 1 at 92. As a final example, suppose the initialreading on meter 42 indicates that the input source under test is atapproximately 8 kilovolts. The input voltage is then applied to the 10KVDC input terminal 46 via cable 34; assume that the 100 VDC outputrange at tap 52 is selected and that a reading of 80 volts is obtainedfrom the differential voltmeter 32. From table 1, it is found that theapplicable multiplier is M=l00. Therefore, E, M'E (i.e. the reading),and E,,, 80 volts X 100 8,000 VDC.

It should be realized that the DC input impedance of the measuringinstrument, such as meter 32, connected to the divider outputconstitutes a shunting resistance across the lower resistors of thedivider. However, the effect thereof is negligible and can be ignored,since potentiometric and differential voltmeters such as meter 32exhibit essentially infinite input impedance when at null. It is alsopointed out that the 100 volt output at 52 is intended primarily for usewith a differential voltmeter, whereas the 10 volt output at 54 servesbest for interfacing to recording instruments via a unity gainamplifier, and the 1 volt output at 56 serves foruse with a precisionpotentiometer measuring instrument.

Turning now to a consideration of the resistor modules 1A-3B and theirdetails of construction, it should be realized that these modulescomprise one of the most important aspects of the invention. The modules1A-3B, forming the high voltage input section of the divider, musthandle the high voltages involved, and their construction has been foundto be critical to realizing the high accuracy (maximum error i 0.01

percent) desired and to solving the problems of corona, leakage current,etc., discussed in the Background and Summary sections, supra.

Reference is now made to FIGS. 4 and showing the construction requiredfor resistor modules 18 and 2B, the module being shown in exploded formbefore assembly in FIG. 4 and as assembled in FIG. 5. The

module comprises an outer insulating shell 110 in the form of a hollow,relatively thin-walled support tube 1 having a square shapedcross-section, as shown in the drawing. The tube is formed of alaminated glass epoxy, such as G-10 which is a special grade siliconeglass epoxy. This is a material having very high volume and surfaceresistivity values. The shell or tube 110 is provided with a pair of endplates 111 and 112 to cover its opposite ends, the plates beingcomprised of the same G-l0 glass epoxy. End plate 111 is dimensioned tofit inside tube 110 while plate 112 is flush with the other end of thetubeQThe plates have centrally located tapped holes 113 and 114, andplate 112 further has a large aperture 115 in the upper right quadranttogether with a small aperture 116 below it. The other end plate 111 isprovided with a large aperture 117 aligned with 115. An insulated rod118 of 6-10 glass epoxy, threaded at both ends, is adapted to bepositioned along the center line of the module and to engage tappedholes 113 and 114.

Each module contains a series connected string of five precision wirewound high resistance resistors 120 each having a small central hole oraperture through which they are mounted on the rod 118 as shown. Theradially extending terminal lugs of the resistors are bent over intooverlapping positions, as best seen in FIG. 4, to be soldered togetherto make the series circuit connection. The modules 1B and 2B feature atrimming potentiometer 121 having a slotted adjustment shaft 122 to bemounted in aperture 116 at the front end of the module. The trimmingpotentiometer is provided so that a pair of modules such as lA-lB canhave their total series resistance set at the required circuit valueduring factory calibration of the assembled divider 30. The modules 1Band 2B are provided with A-MPf LGH* Trademark of AMP Incorporated %Lhigh voltage receptacle-type connectors at each end for safe, positiveelectrical connection of the internal resistor string to external partsof the circuit. These high voltage connectors are shown at 123 and 124,and they are to be mounted in apertures and 117 of the end plates. Theback end plate 11 contains two further apertures through which extend apair of tubes 125 and 126 used to introduce a filling and encapsulatingmaterial into the module after assembly, as will be described.

The modules 1B and 2B are assembled in the'following manner. Theresistors are stacked onthe rod 118 and their terminal lugs are bentover and soldered together. Care is taken here to get a ball-shapedsolder joint with no sharp edges or pointed portions, thereby reducingthe chances of forming corona inside the module. The receptacle-typeconnectors 123 and 124 are bonded in place in the end plates with anysuitable epoxy adhesive, and the potentiometer 121 is similarly bondedinto aperture 116 of end plate 112. The threaded rod 118 carrying theresistor string is placed in engagement in the end pieces, and the backend of the resistor string is attached and terminated to the conductor124 by soldered conductor wire. Appropriate connecting wiring is alsomade to trimming potentiometer 121 to connect it in series to theremaining end of the resistor string and to the other connector 123.Then the outer shell or tube is slid into position over the resistorstring assembly and bonded to end pieces 1 11, 112 with epoxy adhesive.

The module is then potted with an encapsulating resin, preferably aBaker System 37 polyurethane filling medium, which consists of a Vorite63 prepolymer (66 percent by weight) and a Polycin 52 polyol (34 percentby weight). This resin is preheated and vacuum treated to remove all airbubbles and is then pressure injected into filling tube 125, with theother tube 126 serving as an outlet for air displaced by the fillingmedium. The filled module is heat-treated to cure and set up theencapsulant at elevated temperature, and the ends of tubes 125, 126 aretrimmed flush with the outside of the module. The finished module ispainted with an epoxy point, one which is free of carbon black, tomaximize surface resistivity. The module is ready for mounting in thedivider 30. 7

FIGS. 6 and 7 depict the construction for modules 2A and 3A, which isbasically the same as the other module depicted in FIGS. -4 and 5,except that the trimming potentiometer is omitted and the connectorreceptacle arrangement is modified. Parts in FIGS. 6 and 7 which are thesame as in FIGS. 4 and 5 have like reference numerals. The forward endplate in this second type of module has a pair of large apertures 127and 128 positioned as shown in FIG. 6. These apertures receive and mountrespective high voltage connectors 129 and 130, which are an A-MP*LGl-I* Trademark of AMP Incorporated 9&1. connector and an A-MP' LGIITrademark of AMP Incorporated lL connector, respectively. Theseconnectors are fully described in U.S. Pat. No. 2,958,844, issued Nov.1, 1960, to W. A. Smith et al., and assigned to the present assignee.The module of FIGS. 6 and 7 is otherwise identical to the previous one,and is assembled the same way. In FIGS. 6 and 7 the forward terminal lugof the first resistor is wired, internally of the module, in parallel tothe two connector receptacles 129 and 130.

Referring to FIGS. 1, 2 and 8, the manner of assembling the modules toconstruct the voltage divider proper will now be explained. As shown inFIGS. 1 and 2, the divider instrument includes an L-shaped main frame orbase 140 of a tough, resilient molded plastic, on which the resistormodules of the circuit input section are mounted. A back cover shroud142 engages the base 140 to enclose the circuitry. The auxiliary meter42, the ratio selector switch 40, and the output resistance string aredisposed and suitably mounted and wired together behind the upstandingface of the base frame 140, in any suitable fashion, not shown. Turningto FIG. 8, this figure depicts the mounting of the resistor moduleslA-3B above base 140 and within shroud 142, the shroud being indicatedin phantom in the figure. The view is from the back side of theinstrument, with the back wall of shroud 142 considered to be removed.There are provided three mounting boards or plates 144, 146, and 148 onwhich the modules are fastened. The mounting boards are attached to, butseparated from, the main frame and from each other by any suitableinsulated spacer means, not shown. The modules are held in the relativestaggered positions shown (best seen in FIG. 8) and are fastened simplyby screw passing through holes in the mounting boards from below andengaging blind tapped holes in the module shells. The mounting boards144, 146, 148 are composed of the same G-10 glass epoxy material asmentioned previously, and are located with a special silicone coneinsulating varnish (a General Electric varnish SR-98), a high surfaceresistivity material. The same varnish coating is applied to theexterior surfaces of the modules. This coating provides a moisturebarrier or seal between modules and ground plates to combat highhumidity; it also increases the surface resistivity between any moduleand ground, and also from module to module.

Considering again the relative positioning of modules lA-3B as shown byFIG. 8, these positions are such that, upon assembly, voltage gradientsbetween adjacent modules (both in the same plane and above and below inother planes), and even between distant modules, are minimized. Thus,for example, the junction of modules 23 and 3A and the input SKVconductor are all at 'the same potential, as seen from inspecting FIG.3. The corresponding ends of modules 3A and 2B are therefore locatedclose together, but are also separated as far as possible from the backend face of module 3B which is at a quite different potential, and alsofrom the back end face of module 2A for the same reason. The angledpositions of 3A and 2B in the bottom module plane and the skewedposition of module 2A achieve this result.

The modules are suitably interconnected as shown by insulated wirejumper leads terminated at their ends by A-MP* LGl-I* Trademark of AMPIncorporated AL and 11.. pin type high voltage connectors, as seen inFIG. 8. Thus leads 150, I52, and 154 go to the high voltage inputtenninals 44, 46 and 48, respectively. The jumper lead 156 interconnectsmodules 1B and 2A. The jumper leads 158 and 160 at the opposite side ofthe assembly similarly interconnect modules IA and IE on one hand, andmodules 2A and 213 on the other. In each instance the pin connectors ofthe jumpers mate directly with the receptacle high voltage connectorsdisposed in the end walls of the resistor modules.

While wire-wound resistors have been disclosed above, it is contemplatedthat other types of precision resistors, such as metal film resistors,can equally well be employed. Also, the input section could be extrudedto accept higher DC voltages such as 20 KVDC or 30 KVDC, etc. Also, theoutput voltage ranges made available could be varied.

While various embodiments of the invention have been shown anddescribed, it will be understood that various modifications may be made.

I claim:

1. An instrument for coupling a source of voltage in the kilovolt rangeto a low voltage measuring device, comprising:

high voltage input means adapted to be coupled to said source ofvoltage;

impedance module means coupled to said high voltage input means, saidimpedance module means including means for preventing corona dischargeand leakage currents and provided with a plurality of spaced input tapsfor respective different values of high voltage;

output circuit means coupled to said impedance module means, said outputcircuit means including a plurality of output voltage taps for providingrespective different fractions of the voltage appearing on each inputtap;

selectively operable means coupled to said output circuit means forselectively coupling said output voltage taps to said low voltagemeasuring device, an auxiliary voltmeter, and

means for connecting the impedance module means in series with saidauxiliary voltmeter.

2. An instrument for coupling a source of voltage in the kilovolt rangeto a low voltage measuring device, comprising:

high voltage input means adapted to be coupled to said source ofvoltage;

impedance module means coupled to said high voltage input means, saidimpedance module means including means for preventing corona dischargeand leakage currents;

output circuit means coupled to said impedance module means, said outputcircuit means including a plurality of output voltage taps; andselectively operable means coupled to said output circuit means forselectively coupling said output voltage taps to said low voltagemeasuring device, and auxiliary voltmeter means within said instrument;

said selectively operable means further comprising means for selectivelycoupling said auxiliary voltmeter means to said output circuit meanswhile simultaneously disconnecting said output circuit means from saidlow voltage measuring device.

3. A precision circuit for dividing inputs in the 5 kilovolt range,comprising:

including a plurality of series connected precision resistors, andfurther including means for preventing corona discharge and leakage andprovided with a plurality of spaced input taps for respective differentvalues of high voltage;

low voltage output circuit means coupled to said impedance module means,said output circuit means including a plurality of precision low voltageresistors and a plurality of output taps associated therewith forproviding respective different fractions of the voltage appearing oneach input tap;

output terminal means;

selectively operable means coupled to said low voltage output circuitmeans for selectively coupling said output voltage taps to said outputterminal means, an auxiliary voltmeter, and

means for connecting the impedance module means in series with saidauxiliary voltmeter.

4. A precision circuit for dividing inputs in the kilovolt range,comprising:

high voltage input means; 7 impedance module means coupled to said highvoltage input means; said impedance module means including a pluralityof series connected precision resistors, and further including means forpreventing corona discharge and leakage currents,

low voltage output-circuit means coupled to said impedance module means,said output circuit means including a plurality of precision low voltageresistors and a plurality of output taps associated therewith; and

output terminal means;

selectively operable means coupled to said low voltage output circuitmeans for selectively coupling said output voltage taps to said outputterminal means,

and auxiliary volmeter means;

said selectively operable means further comprising means for selectivelycoupling said auxiliar voltmeter means to said low voltage outputcircuit means while simultaneously disconnecting said output taps fromsaid output terminal means.

1. An instrument for coupling a source of voltage in the kilovolt rangeto a low voltage measuring device, comprising: high voltage input meansadapted to be coupled to said source of voltage; impedance module meanscoupled to said high voltage input means, said impedance module meansincluding means for preventing corona discharge and leakage currents andprovided with a plurality of spaced input taps for respective differentvalues of high voltage; output circuit means coupled to said impedancemodule means, said output circuit means including a plurality of outputvoltage taps for providing respective different fractions of the voltageappearing on each input tap; selectively operable means coupled to saidoutput circuit means for selectively coupling said output voltage tapsto said low voltage measuring device, an auxiliary voltmeter, and meansfor connecting the impedance module means in series with said auxiliaryvoltmeter.
 2. An instrument for coupling a source of voltage in thekilovolt range to a low voltage measuring device, comprising: highvoltage input means adapted to be coupled to said source of voltage;impedance module means coupled to said high voltage input means, saidimpedance module means including means for preventinG corona dischargeand leakage currents; output circuit means coupled to said impedancemodule means, said output circuit means including a plurality of outputvoltage taps; and selectively operable means coupled to said outputcircuit means for selectively coupling said output voltage taps to saidlow voltage measuring device, and auxiliary voltmeter means within saidinstrument; said selectively operable means further comprising means forselectively coupling said auxiliary voltmeter means to said outputcircuit means while simultaneously disconnecting said output circuitmeans from said low voltage measuring device.
 3. A precision circuit fordividing inputs in the kilovolt range, comprising: high voltage inputmeans; impedance module means coupled to said high voltage input means;said impedance module means including a plurality of series connectedprecision resistors, and further including means for preventing coronadischarge and leakage and provided with a plurality of spaced input tapsfor respective different values of high voltage; low voltage outputcircuit means coupled to said impedance module means, said outputcircuit means including a plurality of precision low voltage resistorsand a plurality of output taps associated therewith for providingrespective different fractions of the voltage appearing on each inputtap; output terminal means; selectively operable means coupled to saidlow voltage output circuit means for selectively coupling said outputvoltage taps to said output terminal means, an auxiliary voltmeter, andmeans for connecting the impedance module means in series with saidauxiliary voltmeter.
 4. A precision circuit for dividing inputs in thekilovolt range, comprising: high voltage input means; impedance modulemeans coupled to said high voltage input means; said impedance modulemeans including a plurality of series connected precision resistors, andfurther including means for preventing corona discharge and leakagecurrents, low voltage output-circuit means coupled to said impedancemodule means, said output circuit means including a plurality ofprecision low voltage resistors and a plurality of output tapsassociated therewith; and output terminal means; selectively operablemeans coupled to said low voltage output circuit means for selectivelycoupling said output voltage taps to said output terminal means, andauxiliary volmeter means; said selectively operable means furthercomprising means for selectively coupling said auxiliar voltmeter meansto said low voltage output circuit means while simultaneouslydisconnecting said output taps from said output terminal means.